Radio frequency (RF) and microwave filters represent a class of electronic filter, designed to operate on signals in the megahertz to gigahertz frequency ranges. This frequency range is the range used by most broadcast radio, television, wireless communication (cellphones, Wi-Fi, etc.), and thus most RF and microwave devices will include some kind of filtering on the signals transmitted or received. Such filters are commonly used as building blocks for duplexers and diplexers to combine or separate multiple frequency bands.
Cavity filters are the basic circuitry behind a Duplexer. Cavity filters are sharply tuned resonant circuits that allow only certain frequencies to pass. Physically a cavity filter is a resonator inside a conducting “box” with coupling loops at the input and output. Cavity filters are widely used in the 40 MHz to 960 MHz frequency range; well constructed cavity filters are capable of high selectivity even under power loads of at least a megawatt. Higher Q quality factor, as well as increased performance stability at closely spaced (down to 75 kHz) frequencies, can be achieved by increasing the internal volume of the filter cavities. In the microwave range (1000 MHz (or 1 GHz) and higher), cavity filters become more practical in terms of size and a significantly higher quality factor than lumped element, micro-strip, or strip-line resonators and filters.
Cavity filters are fixed filters that lack significant center frequency tuning. In order to accommodate software defined radios large banks of filters may be required because a software defined radio may need to operate under many different sets of parameters.
Some filters are designed with varactors as variable capacitors. Such filters may only handle low power loads and may have poor performance characteristics otherwise.
Consequently, it would be advantageous if an apparatus existed that is suitable for electronically tuning the center frequency of a cavity filter.